1. Field of the Invention
This invention relates to tuned radio antennae finding particular application for a frequency agile radio system and/or for the achievement of improved power efficiency relative to broad-banded antennae with similar dimensions.
2. Prior Art
Many modern radio communication systems, particularly military systems, make use of "frequency agility" technique in which the radio frequency is repetitively changed. The rate of change may typically be at rates up to 1000 per second and it is common to use a large number of frequencies over a wide band. It may be required for example to effect switching between any of the frequencies in the military UHF band or between any of the frequencies in the "tactical VHF" band. It has heretofore generally been the practice to make use of wide band antennae for this purpose. Although many designs for broad banded antennae are well-known, this requirement for operation over a very broad band normally imposes limitations in the power efficiency and/or the voltage standing wave ratio limit, particularly in the tactical VHF band. It has to be borne in mind that antenna design commonly has to be a compromise between a number of diverse requirements, including size, radiation pattern etc.
Heretofore it has not been considered possible to effect switching to tune the antenna to each frequency at the very high switching speeds required with modern frequency agility systems. Tuning of antennae to effect switching is well-known. The Paper by C E Cooper "Airborne Low VHF Antennas" delivered at the 26th Agard Meeting in Munich, November 1973, describes an airborne antenna with a VHF radiator of the capacitive type with six inductances for tuning purposes, which inductances can be short-circuited by vacuum relay switches. The inductances have different magnitudes and by selective switching, tuning to a number of different frequencies is possible. Miniature high vacuum relays are available which have an operating time as short as 100 m.secs. but such relays would have only a very limited life if constantly switched even at only ten changeovers per second. In practice therefore mechanically switched antennae of this nature are not suitable for frequency agility systems.